New stud walls along garage walls

Im looking to put up stud walls around the walls of my garage, in order to convert into into a home gym.

Garage is detatched from the house, with only one brick thick at the back

I have seen people saying that they have used wood battens straight up against the wall, with either a damp proof membrane or vapour barrier, followed by insulation, followed by vapour barrier then platerboard on top.

But then I have also seen comments stating that should leave a gap for air flow.

So questions that I have:

1. Do I need to leave a gap around the walls of the garage (the garage is basically one brick thick, detached from the house), though would prefer not to lose too much space.
   
   
2. With the back of the wood battens, is there any issue with using damp proof membrane near to the walls of the garage, sandwich in insulatation in the gap of the stud walls followed by a vapour barrier?
   
   
3. There are two main brick air vents at the end of the garage, these I assume should go through the stud walls into the plasterboard – but if there is a gap around, do I still need air vents in the stud wall/plasterboard?
   
   
4. The loft has loft board down, if I was to put up a plasterboard ceiling with insulation between the loft board the plasterboard underneath, does it need any sort of air flow or vapour barrier
   
   
5. The garage concrete floor, does this need to have a fake floor to help reduce moisture?
   

The above questions are after I have already put up wood battens on the walls, left holes in the stud walls for the air vents, and put up plasterboard – however, after only a couple of weeks have found that the plasterboard is/was full of moisture, so ended up ripping down the plasterboards.

So not completely sure that I've gone about this correctly, or if there is another issue in the garage causing the moisture.

So I basically just need to know, damp proof membrane, wood battens, insulation, vapour barrier, plasterboard – leave gaps for the air vents, but how do I reduce the amount of moisture in the garage, to stop the plasterboard from soaking up any moisture?

Do I need to seal in that part of the garage with a stud wall across the garage (with door), with an insulated ceiling?

Any help would be appreciated.

 

Property is only 20 years old

But currently the garage door is just a up n over door, which were looking into more insulated garage door.

But the plan was to basically build an inner room with an internal door to get into the gym area